A diaphragm for a speaker is made of synthetic resin. The diaphragm made of synthetic resin has an improved waterproof characteristic and is useful for an automobile speaker, for example. As prior art documental information pertaining to the invention, Unexamined Japanese Patent Publication No. S59-176995 is a publicly known, for instance.
The synthetic resin diaphragm, however, becomes heavy when it is formed thick for getting a similar strength as to a paper diaphragm, making it difficult to emit an audio sound.